[Bug c/56956] ftrapv traps on valid abs-like code

[jasonwucj at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56956

--- Comment #4 from Chung-Ju Wu <jasonwucj at gmail dot com> 2013-04-15 04:18:13 UTC ---
(In reply to comment #2)
> (In reply to comment #1)
> > I think 'x' can not present negative value.
> 
> The unary minus operator is defined for unsigned types. It doesn't produce
> signed overflow.

According to C99 6.5.3.3 Point 3 and C99 6.5 Point 5:

"The result of the unary '-' operator is the negative of its operand."

"... if the result is ... not in the range of representable
 values for its type... the behavior is undefined."

So my understanding is that the evaluation expression '-x' is not
a representable value of 'uint64_t', which is undefined behavior,
resulting abort if -ftrav is issued.

Perhaps my understanding is incorrect? :(